1. Field of the Invention
The present invention is related to the field of radiation detecting devices. More specifically, the present invention is related to methods of determining various elemental concentrations by analysis of the energy spectra of detected radiation.
2. Discussion of the Relevant Art
Radioactive isotopes, which can be present in materials such as earth formations, emit several different types of radiation which can be detected by various types of radiation detectors. In other cases, induced radiation can be emitted from earth formations by introducing radioactive sources into the formations.
For example, gamma ray radiation can be particularly useful for determining the amounts of specific radioactive isotopes which may be present in the materials, because each different isotope emits gamma rays having characteristic energy levels. Amounts of other elements present in the materials can be determined by detection of induced gamma rays, which also have characteristic energy levels. Induced gamma rays are emitted when the elements are present and, for example, neutron sources are introduced into earth formations containing the elements. Radiation detectors have been devised which can determine the energy levels of the gamma rays which are detected.
A type of radiation detector known in the art which can be used to determine the energy levels of the impinging radiation is called a scintillation detector. The scintillation detector typically comprises a single, large crystal composed of a material such as cesium iodide (CsI), sodium iodide (NaI), or bismuth germanate (BGO). Gamma rays entering the crystal cause the crystal to emit a small flash of light, or scintillation. Scintillations typically have a magnitude proportional to the energy level of the gamma ray which caused the scintillation.
The scintillations are optically coupled from the crystal to a photomultiplier tube. The photomultiplier tube emits a voltage pulse which is proportional in amplitude to the magnitude of the scintillation. The voltage pulses can be conducted to various circuits for analysis of the amplitudes of the individual pulses.
Analysis of the numbers of pulses having certain amplitudes corresponding to various energy levels of gamma rays can provide information about the presence of certain elements or isotopes. A graphic representation of the number of pulses occurring with respect to the energy level of the pulses typically displays localized maxima, called "peaks" at several energy levels within the energy range of the scintillation detector, which typically is some portion of the range of 0.1 to 10 million electron volts (MeV), depending on the crystal type and the elements intended to be resolved. The peaks also have a range of energy levels characteristic to the isotope. The range of energy levels is typically defined as the width (on an MeV scale) of the peak at half its maximum value, as will be explained further.
The amplitudes of the voltage pulses are typically analyzed by using a device called a spectral analyzer. The spectral analyzer comprises a pulse height quantizer for measuring the amplitude of each voltage pulse from the photomultiplier, and a storage device for counting the number of voltage pulses of each magnitude determined by the quantizer. Based on the amplitude measurement made by the quantizer, a quantization value called a channel number is assigned to each measured pulse. Each pulse leaving the quantizer increments a particular storage buffer in the storage device corresponding to the channel number determined for each pulse by the quantizer. At the end of any measurement period, the number of events counted in each buffer is used for analysis.
Many spectral analyzers assign channel numbers based on a linear scaling of apparent amplitude of the voltage pulses. Linear scaling means that the channel number is linearly proportional to the amplitude of the voltage pulse, and therefore the apparent energy level of the detected gamma ray which caused the pulse.
One of the drawbacks to linear scaling is that certain elements generate a plurality of energy peaks, spaced closely together in energy level, near the lower end of the energy range of the scintillation detector, which is typically about 100 to 300 thousand electron volts (keV). It is frequently difficult, using linearly scaled channel number assignment, to discriminate between elements having a plurality of peaks in the lower energy range of the detector because an insufficient number of analyzer channels is assigned to the lower energy levels to adequately resolve the peaks.
A method known in the art for improving the peak resolution of the spectral analyzer in the lower end of the detector energy range is disclosed in U.S. Pat. No. 5,289,386, issued to Anderson. The method disclosed in the Anderson patent assigns channel numbers to energy levels using a second order polynomial expression having the form: EQU E=a+bN.sup.2 ( 1)
where a and b are constants which fit a particular crystal and photomultiplier type to the channel analyzer, E is the gamma ray energy level, and N is the channel number. Channel analyzers known in the art typically have 256 channels, numbered zero to 255, which defines the typical range for N.
The method disclosed in the Anderson patent for energy definition of channel numbers still provides inadequate resolution at the lower energy end of the detector range, and unnecessarily high resolution at the upper end of the detector energy range.
It is an object of the present invention to provide a method of assigning channel numbers in a channel analyzer to provide nearly constant numbers of assigned channels for resolving emission spectra occurring throughout the energy detection range of a scintillation counter.